This invention relates generally to compositions and processes for enhancing the separation of hydrocarbon compounds via extractive distillation.
It is difficult to efficiently and economically separate mixtures of organic compounds having similar chemical characteristics, and nearly the same boiling point. Conventional fractional distillation can be expensive and inefficient in this situation, in that large and expensive columns are required, which have large numbers of plates, and utilize high reflux ratios with correspondingly high energy consumption rates.
Extractive distillation is a technique for separating certain close boiling mixtures. In extractive distillation, a high boiling solvent is typically introduced into a distillation zone above the entry point of the lower boiling feed mixture which is to be separated. The high-boiling solvent flows down the distillation zone, and interacts with the feed mixture, to effectively decrease the volatility of some mixture components, typically the more polar feed components, so that the less polar feed components can be distilled overhead, while the solvent and the more polar feed components exit the column with the bottoms fractions. An extractive distillation process has been described in the article entitled xe2x80x9cExtractive Distillation Saves Energyxe2x80x9d by Ian Sucksmith, Chemical Engineering, Jun. 28, 1982, pages 91-95. Other literature sources relating to extractive distillation techniques include the xe2x80x9cHandbook of Separation Techniques for Chemical Engineersxe2x80x9d by Philip A. Schweitzer, McGraw-Hill Book Company, 1979, pages 1-135; Perry""s Chemical Engineers Handbook, 6th Edition, McGraw-Hill Book Company, 1984, pages 13-53.
The separation of various classes of hydrocarbons, such as aromatics, olefins, or cycloalkanes, from other close-boiling hydrocarbons, such as paraffins, by extractive distillation is known in the hydrocarbons industry. In particular, many refinery streams comprise xe2x80x9cBTX streamsxe2x80x9d which comprise close-boiling mixtures of aliphatic hydrocarbons (such as isomers of heptane and octane) and aromatic hydrocarbons (such as benzene, toluene and xylenes). It is known in the industry to use mixtures of solvents such as various polyethylene glycol ethers and water as solvents for extractive distillation of hydrocarbon and/or BTX streams (i.e., xe2x80x9cUdexxe2x80x9d processes, and variations thereof). U.S. Pat. Nos. 3,714,033 and 4,921,581 disclose the use of polyalkylene glycol solvents toward this end.
Since the institution of xe2x80x9cUdexxe2x80x9d technology, efforts have been made to improve its production efficiency and economic performance in hydrocarbon separations by use of other types of solvents for extractive distillation. Morpholine derivatives and N-alkylpyrrolidones were disclosed as extractive distillation solvents, respectively, in U.S. Pat. Nos. 4,081,355 and 4,948,470. U.S. Pat. No. 4,676,872 discloses the use of adiponitrile in combination with other materials such as ethylene carbonate, nitrobenzene, and certain dialkyl phthalate esters, for the separation of xylene isomers. U.S. Pat. No. 4,292,142 discloses the use of phthalic anhydride in combination with other materials such as isophorone, for the separation of ethylbenzene from xylenes.
Other known processes have employed organic sulfones as solvents in extractive distillation processes. U.S. Pat. Nos. 2,033,942 and 2,831,039 described the use of dialkyl sulfones, including dimethylsulfone, in such separations. U.S. Pat. No. 4,401,517 relates to the use of C4-C8 sulfones as selective extractive distillation solvents, the preferred sulfone being di-n-propyl sulfone. U.S. Pat. No. 3,146,190 described the use of sulfolane (tetramethylene sulfone) as a selective extraction solvent for the purification of pyrolysis fuels and catalytically reformed gasolines. U.S. Pat. No. 3,466,346 describes further refinements of sulfolane based extractive distillation processes. U.S. Pat. No. 3,723,256 describes a method for utilizing sulfolane-type solvents in commercial units designed for glycol-based extractants. U.S. Pat. No. 4,053,369 describes methods for optimizing separation efficiencies in sulfolane based extractive distillation processes. Water-sulfolane mixtures are disclosed in U.S. Pat. No. 5,849,982.
Sulfolane is a known solvent for the extractive distillation of aromatic compounds from BTX streams in modem commercial hydrocarbon production. Sulfolane shows a reasonably good selectivity for separating aromatic materials from aliphatic compounds, but, Wu et.al estimate (Chemical Engineering, page 139, March 1998) that for a typical industrial BTX extraction unit, a 1% increase in aromatics recovery would result in savings of up to $100,000 per year. Such savings derive from both decreased need for capital investment, and decreased unit energy requirements (electricity, steam, etc) needed to perform the extractive distillation. Sulfolane also suffers from significant thermal degradation on an annual basis, and therefore requires regular makeup of its volume, which causes significant expense.
Relatively recent approaches to extraction medium modification involve the introduction of additives to sulfolane as a way of enhancing extraction performance. For example, U.S. Pat. No. 5,032,232 describes combinations of N-alkyl-2-thiopyrrolidones and sulfolane for this purpose. U.S. Pat. No. 4,024,028 describes the use of mixtures of dimethyl sulfone, methyl ethyl ketone and sulfolane for extractive distillation of certain hydrocarbon mixtures. Fu-Ming Lee (Chemical Engineering, Vol 105, pg112-118, November 1998) recently described the state of extractive distillation technology, and the use of sulfolane in combination with dialkyl sulfones, including dimethyl sulfone.
Thus, both technical and financial considerations provide a continuing need for developing novel and improved extractive distillation compositions which exhibit advantages (such as higher selectivity and/or capacity/loading) over known solvents for the extractive distillation of mixtures of close-boiling hydrocarbons.
The instant invention provides improved extractive distillation compositions and processes.
In one aspect, the invention relates to an extractive distillation composition for separating a mixture of hydrocarbon compounds comprising:
a) sulfolane and/or a sulfolane derivative, and
b) at least one compatibility agent, wherein the at least one compatibility agent:
i) is selected from materials having a Polar Parameter and a Hydrogen Bonding Parameter such that the extractive distillation composition provides improved separation of the hydrocarbon compounds, as compared to the use of sulfolane and/or a sulfolane derivative without the compatibility agent;
ii) has a boiling point higher than the boiling point of the mixture of hydrocarbon compounds; and
iii) does not comprise an N-substituted-2-thiopyrrolidone compound or residue.
In another aspect, the invention relates to a process for separating hydrocarbon compounds of similar boiling points by extractive distillation, comprising the steps of:
a) contacting a feed mixture with an extractive distillation composition within an extractive distillation zone, wherein the feed mixture comprises at least
i) a first hydrocarbon compound, and
ii) a second hydrocarbon compound; and
b) distilling the feed mixture to at least partially separate the feed mixture into an overhead stream enriched in the first hydrocarbon compound, and a bottoms stream enriched in the second hydrocarbon compound; and
wherein the extractive distillation composition comprises
i) sulfolane and/or a sulfolane derivative, and
ii) at least one compatibility agent, wherein the at least one compatibility agent:
(1) is selected from materials having a Polar Parameter and a Hydrogen Bonding Parameter such that the extractive distillation composition provides improved separation of the hydrocarbon compounds, as compared to the use of sulfolane and/or a sulfolane derivative without the compatibility agent;
(2) has a boiling point higher than the boiling point of the mixture of hydrocarbon compounds; and
(3) does not comprise an N-substituted-2-thiopyrrolidone compound or residue.
In yet another aspect, the invention relates to a process for separating hydrocarbon compounds of similar boiling points by extractive distillation, comprising the steps of:
a) introducing a feed mixture into an extractive distillation zone having an upper portion and a lower portion, wherein the feed mixture comprises at least
i) a first hydrocarbon compound comprising a C7-C10 aliphatic compound, and
ii) a second hydrocarbon compound comprising benzene, toluene, ortho xylene, meta xylene, para xylene, or a mixture thereof;
b) introducing an extractive distillation composition into the upper portion of the extractive distillation zone;
c) distilling the feed mixture to at least partially separate the feed mixture into an overhead stream rich in the first hydrocarbon compound, and a bottoms stream rich in the second hydrocarbon compound;
wherein the extractive distillation composition comprises:
i) about 100 parts by weight of sulfolane or a sulfolane derivative, and
ii) from about 5 to about 35 parts by weight of a compatibility agent or a mixture of compatibility agents, wherein the compatibility agent comprises an unsubstituted or substituted derivative of acetophenone, propiophenone, benzonitrile, a dialkyl phthalate ester, a cinnamate ester, a C6-C12 alkyl ester of acetic, propionic, or butyric acid, a C7-C10 methyl alkyl ketone, isophorone, nitrobenzene, quinoline, or an isoquinoline.
In yet a different aspect, the invention relates to a process for separating a mixture of hydrocarbon compounds by extractive distillation that employs an extractive distillation composition, wherein the improvement comprises the use of an extractive distillation composition comprising:
a) sulfolane and/or a sulfolane derivative, and
b) at least one compatibility agent, wherein the at least one compatibility agent:
i) is selected from materials having a Polar Parameter and a Hydrogen Bonding Parameter such that the extractive distillation composition provides improved separation of the hydrocarbon compounds, as compared to the use of sulfolane and/or a sulfolane derivative without the compatibility agent;
ii) has a boiling point higher than the boiling point of the mixture of hydrocarbon compounds; and
iii) does not comprise an N-substituted-2-thiopyrrolidone compound or residue.
Additional advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention.